Akinori Ozaki

Arsenic and heavy metal contamination in soils under different land use in an estuary in northern Vietnam

Heavy metal contamination of soil and sediment in estuaries warrants study because a healthy estuarine environment, including healthy soil, is important in order to achieve ecological balance and good aquaculture production. The Ba Lat estuary of the Red River is the largest estuary in northern Vietnam and is employed in various land uses. However, the heavy metal contamination of its soil has not yet been reported. The following research was conducted to clarify contamination levels, supply sources, and the effect of land use on heavy metal concentrations in the estuary. Soil samples were collected from the top soil layer of the estuary, and their arsenic (As), chromium (Cr), cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) concentrations were analyzed, as were other soil properties. Most soils in the estuary were loam, silt loam, or sandy loam. The pH was neutral, and the cation exchange capacity ranged from 3.8 to 20 cmol·kg−1. Manganese and iron concentrations averaged 811 µg·g−1 and 1.79%, respectively. The magnitude of the soil heavy metal concentrations decreased in the order of Zn > Pb > Cr > Cu > As > Cd. The concentrations were higher in the riverbed and mangrove forest than in other land-use areas. Except for As, the mean heavy metal concentrations were lower than the permissible levels for agricultural soils in Vietnam. The principal component analyses suggested that soil As, Pb, Zn, Cd, and Cu were of anthropogenic origin, whereas Cr was of non-anthropogenic origin. The spatial distribution of concentration with land use indicated that mangrove forests play an important role in preventing the spread of heavy metals to other land uses and in maintaining the estuarine environment.

Impact of aquatic plants on entrainment phenomena based on wind-induced flow in a closed density stratified water area

It has recently been proposed that water purification could be performed using aquatic plants, since they absorb nutrient salts. The behavior of a substance in a closed water area is affected by turbulent flows from wind-induced flow, which is a mechanical disturbance, and convective flow, which is a thermal disturbance. In a closed density stratified water area, wind-induced flow gives rise to the entrainment phenomenon at the density interface. This phenomenon, which is based on mixing between the upper and lower water layers, lowers the density interface and so affects the water quality. We experimentally investigated the effect of aquatic plants on the turbulent flow from a mechanical disturbance in the closed water area. Results indicated that the presence of floating and submerged plants had a significant effect on the scale of the turbulent entrainment, and that the entrainment velocity depended on the overall Richardson number to the power of −3/2.

The impact of floating water plants on wind-induced flow and wind wave in a closed water area

In some closed water bodies, the fluids are generally stratified based on vertical density differences. The causes of density these differences can be categorized as being due to temperature, salt levels, and turbidity. In addition, the dynamics of water environmental substances are induced by wind-induced turbulence, which is based on mechanical disturbances, and convective flow, which is based on thermal disturbances. Wind-induced flow and wind waves are produced when mechanical disturbances act on the water surface. These effects in turn generate a lowering of the density interface and increases in the upper layer density based on mixing between upper- and lower-layer fluids, which is commonly referred to as an entrainment phenomenon. This entrainment phenomenon greatly affects the dynamics of water quality in a closed water body. The production of turbulent energy at the water surface and its transport towards the interface play an important role in the dynamics of water environmental substances. When the surface of a closed water body is partially covered with floating water plants, it is considered that wind waves and wind current based on wind action body affected by the plants to some extent, especially in small reservoirs and creeks, which have less surface area for the wind to acts on. We have previously reported that the value of entrainment velocity decreases as the coverage rate of floating water plants increases. It is considered that the covered water surface cause a reduction in the turbulent energy produced by wind shear. It is therefore important to clarify the effects of floating water plants on the wind-induced flow and wind waves generated by wind shear. Accordingly we experimentally considered the turbulent characteristics of wind-induced flow and wind waves in a closed water body with floating water plants. The results indicate that the energy of wind-induced flow is affected by the existence of floating water plants and that the energy decrease as the covereage rate of floating water plants increases.

Contamination of Agricultural Soils by Toxic Trace Metals in an Industrial District in Vietnam

Contamination of agricultural soils by toxic trace metals of arsenic (As), lead (Pb), cadmium (Cd), chromium (Cr), copper (Cu) and zinc (Zn) was investigated in an industrial district in Vietnam. In the district, irrigation agriculture is performed through channels, in addition, there are two industrial parks are nearby agricultural land. The purpose of the study is to clarify the magnitude and spatial distribution of the trace metal concentrations, the source of the trace metals, the difference in the concentrations between the two industrial park areas, and usability of soils for agriculture. As a result, the trace metal concentrations were in the order of Cr>Zn>Pb>Cu>As>Cd for the district. No significant differences were observed between the two park areas in concentrations. Cr and Zn concentrations were high near factories in the park areas, and the other concentrations were comparatively high in the areas, showing that the trace metals were supplied from the factories with wastewaters. The correlations observed between Cd, Pb and Zn, between Cr and Zn, and between Cu and as in the concentrations suggested that there were several groups of wastewaters supplying the trace metals. Concentrations of all kinds trace metals exceeded the permissible level for agricultural soils, and the remediation measures to reduce contamination are necessary.

Effect of swaying motion of floating vegetation on wind-induced entrainment phenomenon in closed water body

When floating vegetation grows on the water surface in a closed water body, the vegetation counteracts the magnitude of the wind acting on the water surface, and affects the fluid flow and the water quality variation in it. In this study, in order to investigate the counteraction effect of the luxuriant growth of floating vegetation on the fluid flow and water quality in the closed water body, we conducted hydraulic experiments on wind-induced flow with density stratification using a wind tunnel test tank and imitated vegetation. We used imitated vegetation made from polystyrene foam plates instead of using real floating vegetation. And we experimentally considered the effect of the vegetation using the concept of the wind-induced entrainment phenomenon. In particular, we focused the effect of the occupation rate and the swaying motion of vegetation. The results indicated that the swaying motion of imitated floating vegetation had an effect on the scale of entrainment. In addition the results had the same tendency as the previous study on the wind-induced entrainment phenomenon.

Effect of Floating Vegetation on Wind-Wave in a Closed Water Body

In some closed water body the dynamics of substances in the water are affected by windinduced turbulence based on mechanical disturbances and convective flow based on thermal disturbances. Wind-induced flow and waves are produced when mechanical disturbances act on the water surface. The production of turbulent energy at the water surface and its transit through the density interface play an important role in the dynamics of the water environmental substances contained in the water. When the surface in the closed water body is partially covered with floating vegetation, it is considered that wind waves and wind-induced flow should be to some extent affected by the vegetation, especially in small reservoirs, ponds and creeks that have less surface area upon which the wind can act. Therefore, it is important to clarify the effect of floating vegetation on windinduced flow and wind waves generated by wind shear. We experimentally considered the turbulence characteristics of wind-induced flow and wind waves in the closed water body with floating vegetation using a test tank. This presentation includes a discussion of the impact of floating vegetation on wind-induced flow and wind wave characteristics caused by mechanical disturbance in a closed water body. The result indicate that the energy of wind-induced flow is affected by the existence of floating vegetation and that the wind wave characteristics changed as the floating pattern of the vegetation changed.

Evaluation of water and nutrient deficiency for fertigation using NIR image

For an application of the remote sensing technique on agriculture, an image taken from an artificial satellite is used widely. The image would not be useful in a field-scale precision agriculture due to the limited power of resolution. For the purpose that remote sensing is adopted in a field-scale precision agriculture, the resolution should be increased or an image taken by another technique should be used. In this study, an image of plants was taken by using a digital camera with a band-pass filter. The image was analyzed related with soil moisture and nutrient condition. Broccoli was planted in nine pots different in soil moisture and nutrient condition. An image of plants in a pot was taken by using a digital camera with a band-pass filter (visible range [550, 650, 680, 750nm], NIR range [780, 800nm]) and captured into a personal computer. The digital images were analyzed with computer software to find the reflection characteristics of visible and near infrared rays depending on deficiencies in soil moisture and nutrient. Frequency distribution of brightness was used to evaluate an influence of soil moisture and nutrient deficit from images. The influence due to soil moisture deficit was significantly found in near infrared images (780 and 800 nm) while the difference affected by the nutrient deficit was difficult to be detected with the proposed procedure.

Wind Induced Flow With Some Aquatic Plants In A Closed Density Stratified Water

Recently, a water quality problem in a closed water area, where eutrophication becomes the serious problems, is studied in the several fields. In the recent studies, it is clear that nutrient salts are absorbed into aquatic plants, and water quality purification with aquatic plants is proposed. But in a closed water area, fluid motion is induced only thermal or mechanical disturbance. The behaviors of water environmental material are affected by turbulent flow based on mechanical disturbance of wind induced flow and thermal disturbance of solar radiation. In the closed density stratified water area wind blowing gives the rise to the entrainment phenomenon at density interface. This phenomenon, which is based on the intermixing of the upper and lower water layers, lowers the density interface and affects the water quality. So we considered that the aquatic plants block up thermal and mechanical disturbance in the closed water area. The results indicated that the existence the floating plants and submerged plants contributed to make a significant effect on the scale of the turbulent entrainment. It was also found that the entrainment velocity depended on the Richardson number to the power of –3/2, the stability parameter of density stratified fluid flow.

Daily Cyclic Variations of Dissolved Oxygen in a Closed Water Area with Floating Aquatic Plants

In a closed water area with a poor rate of exchange with external water, such as a reservoir or a creek, eutrophication, lack of oxygen in the bottom layer, and other water quality problems easily arise. In order to preserve and improve the water environment in such a stagnant closed water area, it is essential to clarify the response characteristics of environmental elements to the various biological and physical processes present in a closed water area. In a eutrophicated water area, it is extremely important to comprehend short-term variation such as the daily cyclic variation of DO concentration for preservation and improvement of the water environment. In the present study, we examined how the existences of floating aquatic plants contribute to the disturbances and properties of daily cyclic variation of the DO concentration in a closed water area containing floating aquatic plants from field observations.Field observations dates indicated that when only thermal disturbance acted in a closed water area with floating aquatic plants, the DO concentration based on photosynthesis by solar insolation was higher than that of observation sites without floating aquatic plants. Additionally, water temperature and DO concentration perform a daily cycle of mixed layer formation at night and stratification in the daytime.

Wind Induced Turbulent Entrainment in a Closed Density Stratified Water Area Partially Covered with the Floating Water Plants

Water environmental dynamics in closed water areas with density stratify such as ponds, reservoirs, lakes and estuaries are effected by turbulent flows based on mechanical and thermal disturbances. The thermal disturbance consists of lighting and heating based on solar radiation in the daytime and cooling based on emission at the night. The mechanical disturbance of wind induced flow and wind wave generated by wind action on the surface. We considered the mechanical disturbance of wind induced flow generated by wind action on the surface in the closed water areas. An experimental investigation of turbulent entrainment induced by the mechanical disturbance was carried out in a laboratory wind –wave tank. We considered the effects of occupying rate of the floating water plants to the water surface area on the turbulent entrainment in the closed water area. The results indicated that the existence the floating water plants contributed to make a significant effect on the scale of the turbulent entrainment. It was also found that the entrainment velocity depended on the Richardson number to the power of –3/2, the stability parameter of density stratified fluid flow.